1. Field of the Invention
The present invention relates generally to laser soldering, and more particularly to a method for laser soldering electronic components to a three dimensional circuit board substrate.
2. Disclosure Information
Electronic components are typically connected to circuit boards using either conventional reflow or wave soldering. An alternative and heretofore less widely used method is laser soldering, illustrated in FIG. 1. Laser soldering is often preferred over conventional reflow processing when the substrate is made of a material that is more temperature-sensitive than conventional laminated FR4 glass-epoxy, such as a low melting point polymer or a thin flex circuit.
In conventional laser soldering, an electronic component 10 is placed atop a dielectric substrate 14 with the component terminations 12 resting atop solder pasted mounting pads 24, and a laser beam 70 from a YAG, Nd:YAG, diode, or other laser 50 is directed at one or more mounting pads 24 and/or solder depositions 26, in order to melt and reflow the solder paste 26 to form solder -joints.
However, conventional laser soldering has always been limited to use with those components whose terminations remain visible after the component is mounted atop its respective solder-pasted mounting pads. These include components having conventional J-leads, gull-wing leads, or end terminations (such as in leadless chip components or LCCs) arranged about one or more sides of the component body. However, laser soldering has heretofore not been utilized on components whose terminations are arranged on an underside surface of the component, such as ball grid arrays (BGAs), pin grid arrays (PGAs), and the like. This is because the standard practice in laser soldering is to direct the laser beam 70 in a line-of-sight manner from a position above the substrate top surface 16 on which the component 10 is mounted, thus precluding its use on components whose terminations are "hidden" beneath the underside of the component when the component is mounted on its solder-pasted mounting pads. Thus, when a circuit board/substrate contains, for instance, J-lead, gull-wing, and BGA/PGA components and it is desired to use laser soldering, typical practice in this case would be to laser solder only the J-lead and gull-wing components, and to separately solder the BGAs/PGAs using a separate reflow process either before or after the laser soldering process. This requirement of a separate reflow step presents a significant drawback to the prospect of using laser soldering, not only because it requires the use of two separate soldering steps (i.e., reflow and laser soldering), but also because it typically requires the use of two separate solder pastes (i.e., one having a first melting point for the J-lead/gull-wing components, and another having a different melting point for the BGA/PGA components).
Because space is limited, housings for automotive vehicle components are continuously being reduced. Commonly, housings have several circuit boards contained therein. The multiple circuit boards take a substantial amount of space. However, further reduction in space of electrical components is desirable.
It also would be desirable to provide a method for using laser soldering with both (1) BGAs, PGAs, and other components having one or more terminations on an underside surface or a an inside surface thereof, and (2) J-lead, gull-wing, endterminated, and other such components.